typically yes, because the readers almost always have flat plane antennas (printed PCBs, wire wound antennas, etc.) and the x-series is a cylinder which couples in a very specific way with these types of antennas - you must present an x-series to lay perpendicular across the antenna trace path of the reader.
the flex is also a flat plane antenna, so coupling is more natural with a flat plane reader antenna, and is typically less “fiddly” when it comes to placement (see my comment on performance in reply to your other post).